Selecting the Proper Size Carb by Dan JonesThere are a number of sizing formulas that can be used to estimate the best

flow rating for a given application. The standard equation relating engine

size and required carb flow is:

where:

DISP = engine displacement in cubic inches

CFM = required carb flow in cubic feet per minute

RPM = maximum engine speed in revolutions per minute

VE = volumetric efficiency (dimensionless, 1.0 = 100%)

1728 = conversion factor between cubic inches and cubic feet

= 12*12*12

2 = conversion factor for four stroke engine

This equation can be simplified to:

DISP * RPM * VE

CFM = ---------------

3456

Note this sizing formula is simply a relationship between cylinder volume and

the flow required to fill that volume at a given engine speed. Also note,

for a four stroke engine, displacement is divided by two because an intake

stroke occurs every other revolution. Another implicit assumption is that

the carb is mounted on a plenum style intake. Independent runner intakes will

require much different sizing. While it's easy to determine displacement and

maximum rpm, you'll probably have to guess at the third variable, volumetric

efficiency (VE), unless you have access to a dyno. Volumetric efficiency is

a simply a measure of how efficiently an engine fills its cylinders. A stock,

low performance, street engine may have a VE between 0.7 and 0.8. High

performance street engines may fall between 0.8 and 1.0, while highly tuned

race engines can have VE's exceeding 1.0, perhaps as high as 1.25.

One other thing to understand when using the formula above is that a carb will

only flow in the presence of a pressure differential. On one side of the carb

there is atmospheric pressure and on the other side is manifold pressure

(usually referred to as manifold vacuum since it is typically lower than

atmospheric pressure). Since engines vary in their manifold vacuum

characteristics, a standardized pressure differential was established to

provide a meaningful comparison for different carbs. Before this standard,

venturi size was used for comparison. The standard for four barrel carbs is

a pressure differential equal to 1.5 inches of mercury (Hg). What this means

is a 4 barrel carb rated at 500 CFM will flow 500 CFM of air, at wide open

throttle, when a pressure differential of 1.5 In Hg is applied across it.

When installed on an engine, this same carb may flow more or less. Two barrel

carbs are usually rated at a different pressure differential (3.0 In Hg). The

reason for this is primarily historical. When 4 barrel carbs first came into

popular use, the vacuum pumps used to rate 2 barrel carbs were unable to pull

the same pressure differential across a 4 barrel carb, so 4 barrels were rated

at a lower pressure drop.

Flow ratings from one standard can be related to flow ratings from another

standard. For 2 and 4 barrel carbs:

Flow @ 1.5 In Hg = CFM Rating @ 3.0 In Hg

----------------------

SQRT(3.0/1.5)

Which is approximately:

Flow @ 1.5 In Hg = CFM Rating @ 3.0 In Hg

----------------------

1.414

This relationship is derived from the fact that, for incompressible flow, the

volumetric flow rate through a venturi is proportional to the square root of

the pressure differential:

Q = K1*A2*SQRT(2*Gc/Rho)*SQRT(P1-P2)

or more simply:

Q = K2*SQRT(P1-P2)

where:

Q = volumetric flow rate

K1 = flow coefficient

A2 = downstream area of the venturi

Gc = gravitational constant

Rho = density

P1 = inlet pressure

P2 = pressure at venturi minimum area

K2 = K1*A2*SQRT(2*Gc/Rho)

Computing the relationship for volumetric flow rate at the two flow

differentials and equating yields the conversion formula. Note the implicit

assumption that the flow coefficient does not change (it can).

As an example of using the sizing formula, let's say we have a modified 4.1

liter (252 cubic inches) Buick V6 with a VE of 0.9 and we plan to turn no

more than 6400 rpm. Plugging our numbers into the formula yields a

theoretical estimate of:

252 * 6400 * 0.9

CFM = ----------------

3456

= 420 CFM

In practice, Joe Murawski of the Wedge list runs a 4.1L Buick in his Triumph

TR7 and has tried a variety of carbs, in sizes ranging from a Holley 390 to

a 785 CFM Quadrajet, settling on a 500 CFM Edelbrock/Carter AFB as providing

the best power and driveability. His carb choice is somewhat larger than that

predicted. For reasons discussed below, we'll see this is not unusual.

While the formula above may yield useful estimates, it is not necessarily the

ideal it is often portrayed to be. If you have a carb that can flow 500 CFM

in the same application and still properly atomize the fuel, it should make

more power than a 400 CFM carb. From this perspective, larger is better.

Ideally, a carb would present zero restriction to the intake stroke. Such a

carb would have an infinite flow rating. Unfortunately, carbs require a

pressure differential to properly mix fuel with air, which is why carb sizing

is important (and why the above formula is useful). Keep increasing the size

of a carb and, at some point, the booster venturis will not properly atomize

the fuel/air mixture and droplets of liquid fuel will be pulled into the

cylinders. Not only is this bad for performance, it's also hard on the engine.

The liquid fuel tends to wash oil off the cylinder walls, increasing ring

and bore wear. This is a particular problem with engines using large overlap

cams, since they provide lower vacuum levels. When using a larger carb and

cam, proper tuning (carb and ignition) becomes more important.

It's important to understand the basic sizing formula is just a guideline.

It ignores a number of important factors such as manifold design, cam timing,

vehicle weight, gearing, transmission type, intended usage, etc. Furthermore,

it ignores important differences in carb design like venturi efficiency, bore

layout, and secondary style and method of actuation. In practice, I have found

that the above formula applies mainly to square bore carbs with non-air valve

secondaries (Holleys, Autolites), and even then it can be conservative for a

performance application. It typically yields a compromise of fuel efficiency

and power.

Using a dual plane, divided plenum, intake usually allows the use of a carb

with a larger CFM rating than with a single plane, open plenum, intake. This

is because the divider cuts the effective plenum volume in half, increasing

the signal to the boosters. Because of this fact, Edelbrock suggests

multiplying the CFM predicted by the basic sizing formula by 1.1 to 1.3 for

single plane manifolds and by 1.2 to 1.5 for dual planes.

As another example, consider the engine I'm currently running in my Pantera.

It's a 351C Ford with Aussie 2V quench heads, 1 3/4" headers, and a single

plane, open plenum, Weiand Xcelerator intake manifold. Since I retained the

stock cast pistons, I chose a cam with a shift point of 6000 rpm. As a

guess, pick 0.9 for the VE. Since the Pantera is relatively light with short

gearing, pick the high side of the range for K (the intake factor):

K*DISP * RPM * VE 1.3*351*6000*0.9

CFM = ----------------- = ----------------

3456 3456

= 713 CFM

This agrees with real world Pantera club experience with Holleys on street

modified 351C's. 600 CFM carbs provide good throttle response and fuel economy

but give up 20+ peak horsepower to 750 carbs. On the downside, the 750 hurts

fuel economy and has poorer throttle response. The happy medium is probably

somewhere in between. Note we're referring to stock Holley carbs here, not

custom models with milled choke horns, thinned butterflies, and improved

booster designs. Those modified carbs can flow more mixture, while providing

adequate atomization.

I chose a Holley 735 from a 428CJ application which seems to work well. This

carb, while flowing nearly as much as a 750, has a venturi cluster design that

provides a stronger signal. Throttle response and fuel economy are relatively

good (20+ mpg on the highway), without incurring a noticeable power penalty.

Two other important considerations are bore layout and method of secondary

actuation. Carbs with air valve secondaries (Carters and Rochester Quadrajets), especially those with spread bore layouts (Thermo Quads, Quadrajets), can

usually be sized larger than square bore Holleys and Autolites. This is

because the smaller primaries increase the flow speed through the boosters,

providing better atomization, while the air valve secondaries passively

restrict air flow until the engine can handle it. Taking these two factors

into consideration, Vizard suggests the following two rules of thumb for street performance engines where power is more important than fuel economy. For air

valve secondary carbs with an upper rpm limit of 6000 rpm, use:

CFM = 2.3 * DISP

For square bore non-air valve secondary carbs use:

CFM = 2.0 * DISP

For engine speeds above 6000 rpm, multiply by the ratio of maximum rpm to

6000 rpm. Note the second formula yields 702 CFM for my 351C example, which

is close to the basic sizing formula with the intake manifold correction

factor applied.

As an extreme example, I've successfully used a 750 CFM Quadrajet on a

relatively stock 231 cubic inch Buick V6. With the Qjet, it got slightly

better fuel economy than the previous 2 barrel carb (due to the small

primaries) and had noticeably more power (due to the huge secondaries). The

driveability of the carb was fine with no bogs or flat spots. On the V6, I'm

sure it never pulled anywhere near the 750 CFM rating but it did pull what

it required. You could never put a Holley 750 on a little low compression V6

and expect to make it work. The air valve secondaries allow the use of much

larger CFM ratings without incurring driveability problems. There is a price

to be paid however. Even when wide open, air valve secondaries are slightly

more restrictive to airflow than non-air valve secondaries.

While these formulas should help you choose a carb flow rating, nothing beats

trail and error. Also, once you have a carb installed, you can determine how

restrictive it is by using a vacuum gauge to measure the difference between

atmospheric pressure and the pressure under the carb. With the air cleaner

removed, the air above the carb will be essentially atmospheric. If there's

any difference between it and the pressure sensed under the carb, it's due

to the carb. The higher the difference, the greater the restriction.

Measurements should be made at wide open throttle and 0.7 inches of mercury

is considered non-restrictive.

Dan Jones